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391 lines
27 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>C Dialect Options - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Invoking-GCC.html#Invoking-GCC" title="Invoking GCC">
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<link rel="prev" href="Invoking-G_002b_002b.html#Invoking-G_002b_002b" title="Invoking G++">
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<link rel="next" href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options" title="C++ Dialect Options">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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</head>
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<body>
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<div class="node">
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<a name="C-Dialect-Options"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="C_002b_002b-Dialect-Options.html#C_002b_002b-Dialect-Options">C++ Dialect Options</a>,
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Previous: <a rel="previous" accesskey="p" href="Invoking-G_002b_002b.html#Invoking-G_002b_002b">Invoking G++</a>,
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Up: <a rel="up" accesskey="u" href="Invoking-GCC.html#Invoking-GCC">Invoking GCC</a>
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<hr>
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</div>
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<h3 class="section">3.4 Options Controlling C Dialect</h3>
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<p><a name="index-dialect-options-102"></a><a name="index-language-dialect-options-103"></a><a name="index-options_002c-dialect-104"></a>
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The following options control the dialect of C (or languages derived
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from C, such as C++, Objective-C and Objective-C++) that the compiler
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accepts:
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<a name="index-ANSI-support-105"></a>
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<a name="index-ISO-support-106"></a>
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<dl><dt><code>-ansi</code><dd><a name="index-ansi-107"></a>In C mode, this is equivalent to <samp><span class="option">-std=c90</span></samp>. In C++ mode, it is
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equivalent to <samp><span class="option">-std=c++98</span></samp>.
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<p>This turns off certain features of GCC that are incompatible with ISO
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C90 (when compiling C code), or of standard C++ (when compiling C++ code),
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such as the <code>asm</code> and <code>typeof</code> keywords, and
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predefined macros such as <code>unix</code> and <code>vax</code> that identify the
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type of system you are using. It also enables the undesirable and
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rarely used ISO trigraph feature. For the C compiler,
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it disables recognition of C++ style ‘<samp><span class="samp">//</span></samp>’ comments as well as
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the <code>inline</code> keyword.
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<p>The alternate keywords <code>__asm__</code>, <code>__extension__</code>,
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<code>__inline__</code> and <code>__typeof__</code> continue to work despite
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<samp><span class="option">-ansi</span></samp>. You would not want to use them in an ISO C program, of
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course, but it is useful to put them in header files that might be included
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in compilations done with <samp><span class="option">-ansi</span></samp>. Alternate predefined macros
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such as <code>__unix__</code> and <code>__vax__</code> are also available, with or
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without <samp><span class="option">-ansi</span></samp>.
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<p>The <samp><span class="option">-ansi</span></samp> option does not cause non-ISO programs to be
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rejected gratuitously. For that, <samp><span class="option">-Wpedantic</span></samp> is required in
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addition to <samp><span class="option">-ansi</span></samp>. See <a href="Warning-Options.html#Warning-Options">Warning Options</a>.
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<p>The macro <code>__STRICT_ANSI__</code> is predefined when the <samp><span class="option">-ansi</span></samp>
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option is used. Some header files may notice this macro and refrain
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from declaring certain functions or defining certain macros that the
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ISO standard doesn't call for; this is to avoid interfering with any
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programs that might use these names for other things.
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<p>Functions that are normally built in but do not have semantics
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defined by ISO C (such as <code>alloca</code> and <code>ffs</code>) are not built-in
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functions when <samp><span class="option">-ansi</span></samp> is used. See <a href="Other-Builtins.html#Other-Builtins">Other built-in functions provided by GCC</a>, for details of the functions
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affected.
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<br><dt><code>-std=</code><dd><a name="index-std-108"></a>Determine the language standard. See <a href="Standards.html#Standards">Language Standards Supported by GCC</a>, for details of these standard versions. This option
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is currently only supported when compiling C or C++.
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<p>The compiler can accept several base standards, such as ‘<samp><span class="samp">c90</span></samp>’ or
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‘<samp><span class="samp">c++98</span></samp>’, and GNU dialects of those standards, such as
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‘<samp><span class="samp">gnu90</span></samp>’ or ‘<samp><span class="samp">gnu++98</span></samp>’. When a base standard is specified, the
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compiler accepts all programs following that standard plus those
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using GNU extensions that do not contradict it. For example,
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<samp><span class="option">-std=c90</span></samp> turns off certain features of GCC that are
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incompatible with ISO C90, such as the <code>asm</code> and <code>typeof</code>
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keywords, but not other GNU extensions that do not have a meaning in
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ISO C90, such as omitting the middle term of a <code>?:</code>
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expression. On the other hand, when a GNU dialect of a standard is
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specified, all features supported by the compiler are enabled, even when
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those features change the meaning of the base standard. As a result, some
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strict-conforming programs may be rejected. The particular standard
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is used by <samp><span class="option">-Wpedantic</span></samp> to identify which features are GNU
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extensions given that version of the standard. For example
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<samp><span class="option">-std=gnu90 -Wpedantic</span></samp> warns about C++ style ‘<samp><span class="samp">//</span></samp>’
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comments, while <samp><span class="option">-std=gnu99 -Wpedantic</span></samp> does not.
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<p>A value for this option must be provided; possible values are
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<dl>
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<dt>‘<samp><span class="samp">c90</span></samp>’<dt>‘<samp><span class="samp">c89</span></samp>’<dt>‘<samp><span class="samp">iso9899:1990</span></samp>’<dd>Support all ISO C90 programs (certain GNU extensions that conflict
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with ISO C90 are disabled). Same as <samp><span class="option">-ansi</span></samp> for C code.
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<br><dt>‘<samp><span class="samp">iso9899:199409</span></samp>’<dd>ISO C90 as modified in amendment 1.
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<br><dt>‘<samp><span class="samp">c99</span></samp>’<dt>‘<samp><span class="samp">c9x</span></samp>’<dt>‘<samp><span class="samp">iso9899:1999</span></samp>’<dt>‘<samp><span class="samp">iso9899:199x</span></samp>’<dd>ISO C99. This standard is substantially completely supported, modulo
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bugs and floating-point issues
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(mainly but not entirely relating to optional C99 features from
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Annexes F and G). See
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<a href="http://gcc.gnu.org/c99status.html">http://gcc.gnu.org/c99status.html</a><!-- /@w --> for more information. The
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names ‘<samp><span class="samp">c9x</span></samp>’ and ‘<samp><span class="samp">iso9899:199x</span></samp>’ are deprecated.
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<br><dt>‘<samp><span class="samp">c11</span></samp>’<dt>‘<samp><span class="samp">c1x</span></samp>’<dt>‘<samp><span class="samp">iso9899:2011</span></samp>’<dd>ISO C11, the 2011 revision of the ISO C standard. This standard is
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substantially completely supported, modulo bugs, floating-point issues
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(mainly but not entirely relating to optional C11 features from
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Annexes F and G) and the optional Annexes K (Bounds-checking
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interfaces) and L (Analyzability). The name ‘<samp><span class="samp">c1x</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">gnu90</span></samp>’<dt>‘<samp><span class="samp">gnu89</span></samp>’<dd>GNU dialect of ISO C90 (including some C99 features).
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<br><dt>‘<samp><span class="samp">gnu99</span></samp>’<dt>‘<samp><span class="samp">gnu9x</span></samp>’<dd>GNU dialect of ISO C99. The name ‘<samp><span class="samp">gnu9x</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">gnu11</span></samp>’<dt>‘<samp><span class="samp">gnu1x</span></samp>’<dd>GNU dialect of ISO C11. This is the default for C code.
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The name ‘<samp><span class="samp">gnu1x</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">c++98</span></samp>’<dt>‘<samp><span class="samp">c++03</span></samp>’<dd>The 1998 ISO C++ standard plus the 2003 technical corrigendum and some
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additional defect reports. Same as <samp><span class="option">-ansi</span></samp> for C++ code.
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<br><dt>‘<samp><span class="samp">gnu++98</span></samp>’<dt>‘<samp><span class="samp">gnu++03</span></samp>’<dd>GNU dialect of <samp><span class="option">-std=c++98</span></samp>. This is the default for
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C++ code.
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<br><dt>‘<samp><span class="samp">c++11</span></samp>’<dt>‘<samp><span class="samp">c++0x</span></samp>’<dd>The 2011 ISO C++ standard plus amendments.
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The name ‘<samp><span class="samp">c++0x</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">gnu++11</span></samp>’<dt>‘<samp><span class="samp">gnu++0x</span></samp>’<dd>GNU dialect of <samp><span class="option">-std=c++11</span></samp>.
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The name ‘<samp><span class="samp">gnu++0x</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">c++14</span></samp>’<dt>‘<samp><span class="samp">c++1y</span></samp>’<dd>The 2014 ISO C++ standard plus amendments.
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The name ‘<samp><span class="samp">c++1y</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">gnu++14</span></samp>’<dt>‘<samp><span class="samp">gnu++1y</span></samp>’<dd>GNU dialect of <samp><span class="option">-std=c++14</span></samp>.
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The name ‘<samp><span class="samp">gnu++1y</span></samp>’ is deprecated.
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<br><dt>‘<samp><span class="samp">c++1z</span></samp>’<dd>The next revision of the ISO C++ standard, tentatively planned for
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2017. Support is highly experimental, and will almost certainly
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change in incompatible ways in future releases.
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<br><dt>‘<samp><span class="samp">gnu++1z</span></samp>’<dd>GNU dialect of <samp><span class="option">-std=c++1z</span></samp>. Support is highly experimental,
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and will almost certainly change in incompatible ways in future
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releases.
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</dl>
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<br><dt><code>-fgnu89-inline</code><dd><a name="index-fgnu89_002dinline-109"></a>The option <samp><span class="option">-fgnu89-inline</span></samp> tells GCC to use the traditional
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GNU semantics for <code>inline</code> functions when in C99 mode.
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See <a href="Inline.html#Inline">An Inline Function is As Fast As a Macro</a>.
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Using this option is roughly equivalent to adding the
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<code>gnu_inline</code> function attribute to all inline functions
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(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>).
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<p>The option <samp><span class="option">-fno-gnu89-inline</span></samp> explicitly tells GCC to use the
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C99 semantics for <code>inline</code> when in C99 or gnu99 mode (i.e., it
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specifies the default behavior).
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This option is not supported in <samp><span class="option">-std=c90</span></samp> or
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<samp><span class="option">-std=gnu90</span></samp> mode.
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<p>The preprocessor macros <code>__GNUC_GNU_INLINE__</code> and
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<code>__GNUC_STDC_INLINE__</code> may be used to check which semantics are
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in effect for <code>inline</code> functions. See <a href="../cpp/Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined Macros</a>.
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<br><dt><code>-aux-info </code><var>filename</var><dd><a name="index-aux_002dinfo-110"></a>Output to the given filename prototyped declarations for all functions
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declared and/or defined in a translation unit, including those in header
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files. This option is silently ignored in any language other than C.
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<p>Besides declarations, the file indicates, in comments, the origin of
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each declaration (source file and line), whether the declaration was
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implicit, prototyped or unprototyped (‘<samp><span class="samp">I</span></samp>’, ‘<samp><span class="samp">N</span></samp>’ for new or
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‘<samp><span class="samp">O</span></samp>’ for old, respectively, in the first character after the line
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number and the colon), and whether it came from a declaration or a
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definition (‘<samp><span class="samp">C</span></samp>’ or ‘<samp><span class="samp">F</span></samp>’, respectively, in the following
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character). In the case of function definitions, a K&R-style list of
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arguments followed by their declarations is also provided, inside
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comments, after the declaration.
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<br><dt><code>-fallow-parameterless-variadic-functions</code><dd><a name="index-fallow_002dparameterless_002dvariadic_002dfunctions-111"></a>Accept variadic functions without named parameters.
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<p>Although it is possible to define such a function, this is not very
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useful as it is not possible to read the arguments. This is only
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supported for C as this construct is allowed by C++.
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<br><dt><code>-fno-asm</code><dd><a name="index-fno_002dasm-112"></a>Do not recognize <code>asm</code>, <code>inline</code> or <code>typeof</code> as a
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keyword, so that code can use these words as identifiers. You can use
|
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the keywords <code>__asm__</code>, <code>__inline__</code> and <code>__typeof__</code>
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instead. <samp><span class="option">-ansi</span></samp> implies <samp><span class="option">-fno-asm</span></samp>.
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<p>In C++, this switch only affects the <code>typeof</code> keyword, since
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<code>asm</code> and <code>inline</code> are standard keywords. You may want to
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use the <samp><span class="option">-fno-gnu-keywords</span></samp> flag instead, which has the same
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effect. In C99 mode (<samp><span class="option">-std=c99</span></samp> or <samp><span class="option">-std=gnu99</span></samp>), this
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switch only affects the <code>asm</code> and <code>typeof</code> keywords, since
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<code>inline</code> is a standard keyword in ISO C99.
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<br><dt><code>-fno-builtin</code><dt><code>-fno-builtin-</code><var>function</var><dd><a name="index-fno_002dbuiltin-113"></a><a name="index-built_002din-functions-114"></a>Don't recognize built-in functions that do not begin with
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‘<samp><span class="samp">__builtin_</span></samp>’ as prefix. See <a href="Other-Builtins.html#Other-Builtins">Other built-in functions provided by GCC</a>, for details of the functions affected,
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including those which are not built-in functions when <samp><span class="option">-ansi</span></samp> or
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<samp><span class="option">-std</span></samp> options for strict ISO C conformance are used because they
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do not have an ISO standard meaning.
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<p>GCC normally generates special code to handle certain built-in functions
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more efficiently; for instance, calls to <code>alloca</code> may become single
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instructions which adjust the stack directly, and calls to <code>memcpy</code>
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may become inline copy loops. The resulting code is often both smaller
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and faster, but since the function calls no longer appear as such, you
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|
|
cannot set a breakpoint on those calls, nor can you change the behavior
|
||
|
|
of the functions by linking with a different library. In addition,
|
||
|
|
when a function is recognized as a built-in function, GCC may use
|
||
|
|
information about that function to warn about problems with calls to
|
||
|
|
that function, or to generate more efficient code, even if the
|
||
|
|
resulting code still contains calls to that function. For example,
|
||
|
|
warnings are given with <samp><span class="option">-Wformat</span></samp> for bad calls to
|
||
|
|
<code>printf</code> when <code>printf</code> is built in and <code>strlen</code> is
|
||
|
|
known not to modify global memory.
|
||
|
|
|
||
|
|
<p>With the <samp><span class="option">-fno-builtin-</span><var>function</var></samp> option
|
||
|
|
only the built-in function <var>function</var> is
|
||
|
|
disabled. <var>function</var> must not begin with ‘<samp><span class="samp">__builtin_</span></samp>’. If a
|
||
|
|
function is named that is not built-in in this version of GCC, this
|
||
|
|
option is ignored. There is no corresponding
|
||
|
|
<samp><span class="option">-fbuiltin-</span><var>function</var></samp> option; if you wish to enable
|
||
|
|
built-in functions selectively when using <samp><span class="option">-fno-builtin</span></samp> or
|
||
|
|
<samp><span class="option">-ffreestanding</span></samp>, you may define macros such as:
|
||
|
|
|
||
|
|
<pre class="smallexample"> #define abs(n) __builtin_abs ((n))
|
||
|
|
#define strcpy(d, s) __builtin_strcpy ((d), (s))
|
||
|
|
</pre>
|
||
|
|
<br><dt><code>-fhosted</code><dd><a name="index-fhosted-115"></a><a name="index-hosted-environment-116"></a>
|
||
|
|
Assert that compilation targets a hosted environment. This implies
|
||
|
|
<samp><span class="option">-fbuiltin</span></samp>. A hosted environment is one in which the
|
||
|
|
entire standard library is available, and in which <code>main</code> has a return
|
||
|
|
type of <code>int</code>. Examples are nearly everything except a kernel.
|
||
|
|
This is equivalent to <samp><span class="option">-fno-freestanding</span></samp>.
|
||
|
|
|
||
|
|
<br><dt><code>-ffreestanding</code><dd><a name="index-ffreestanding-117"></a><a name="index-hosted-environment-118"></a>
|
||
|
|
Assert that compilation targets a freestanding environment. This
|
||
|
|
implies <samp><span class="option">-fno-builtin</span></samp>. A freestanding environment
|
||
|
|
is one in which the standard library may not exist, and program startup may
|
||
|
|
not necessarily be at <code>main</code>. The most obvious example is an OS kernel.
|
||
|
|
This is equivalent to <samp><span class="option">-fno-hosted</span></samp>.
|
||
|
|
|
||
|
|
<p>See <a href="Standards.html#Standards">Language Standards Supported by GCC</a>, for details of
|
||
|
|
freestanding and hosted environments.
|
||
|
|
|
||
|
|
<br><dt><code>-fopenacc</code><dd><a name="index-fopenacc-119"></a><a name="index-OpenACC-accelerator-programming-120"></a>Enable handling of OpenACC directives <code>#pragma acc</code> in C/C++ and
|
||
|
|
<code>!$acc</code> in Fortran. When <samp><span class="option">-fopenacc</span></samp> is specified, the
|
||
|
|
compiler generates accelerated code according to the OpenACC Application
|
||
|
|
Programming Interface v2.0 <a href="http://www.openacc.org/">http://www.openacc.org/</a><!-- /@w -->. This option
|
||
|
|
implies <samp><span class="option">-pthread</span></samp>, and thus is only supported on targets that
|
||
|
|
have support for <samp><span class="option">-pthread</span></samp>.
|
||
|
|
|
||
|
|
<p>Note that this is an experimental feature, incomplete, and subject to
|
||
|
|
change in future versions of GCC. See
|
||
|
|
<a href="https://gcc.gnu.org/wiki/OpenACC">https://gcc.gnu.org/wiki/OpenACC</a><!-- /@w --> for more information.
|
||
|
|
|
||
|
|
<br><dt><code>-fopenmp</code><dd><a name="index-fopenmp-121"></a><a name="index-OpenMP-parallel-122"></a>Enable handling of OpenMP directives <code>#pragma omp</code> in C/C++ and
|
||
|
|
<code>!$omp</code> in Fortran. When <samp><span class="option">-fopenmp</span></samp> is specified, the
|
||
|
|
compiler generates parallel code according to the OpenMP Application
|
||
|
|
Program Interface v4.0 <a href="http://www.openmp.org/">http://www.openmp.org/</a><!-- /@w -->. This option
|
||
|
|
implies <samp><span class="option">-pthread</span></samp>, and thus is only supported on targets that
|
||
|
|
have support for <samp><span class="option">-pthread</span></samp>. <samp><span class="option">-fopenmp</span></samp> implies
|
||
|
|
<samp><span class="option">-fopenmp-simd</span></samp>.
|
||
|
|
|
||
|
|
<br><dt><code>-fopenmp-simd</code><dd><a name="index-fopenmp_002dsimd-123"></a><a name="index-OpenMP-SIMD-124"></a><a name="index-SIMD-125"></a>Enable handling of OpenMP's SIMD directives with <code>#pragma omp</code>
|
||
|
|
in C/C++ and <code>!$omp</code> in Fortran. Other OpenMP directives
|
||
|
|
are ignored.
|
||
|
|
|
||
|
|
<br><dt><code>-fcilkplus</code><dd><a name="index-fcilkplus-126"></a><a name="index-Enable-Cilk-Plus-127"></a>Enable the usage of Cilk Plus language extension features for C/C++.
|
||
|
|
When the option <samp><span class="option">-fcilkplus</span></samp> is specified, enable the usage of
|
||
|
|
the Cilk Plus Language extension features for C/C++. The present
|
||
|
|
implementation follows ABI version 1.2. This is an experimental
|
||
|
|
feature that is only partially complete, and whose interface may
|
||
|
|
change in future versions of GCC as the official specification
|
||
|
|
changes. Currently, all features but <code>_Cilk_for</code> have been
|
||
|
|
implemented.
|
||
|
|
|
||
|
|
<br><dt><code>-fgnu-tm</code><dd><a name="index-fgnu_002dtm-128"></a>When the option <samp><span class="option">-fgnu-tm</span></samp> is specified, the compiler
|
||
|
|
generates code for the Linux variant of Intel's current Transactional
|
||
|
|
Memory ABI specification document (Revision 1.1, May 6 2009). This is
|
||
|
|
an experimental feature whose interface may change in future versions
|
||
|
|
of GCC, as the official specification changes. Please note that not
|
||
|
|
all architectures are supported for this feature.
|
||
|
|
|
||
|
|
<p>For more information on GCC's support for transactional memory,
|
||
|
|
See <a href="../libitm/Enabling-libitm.html#Enabling-libitm">The GNU Transactional Memory Library</a>.
|
||
|
|
|
||
|
|
<p>Note that the transactional memory feature is not supported with
|
||
|
|
non-call exceptions (<samp><span class="option">-fnon-call-exceptions</span></samp>).
|
||
|
|
|
||
|
|
<br><dt><code>-fms-extensions</code><dd><a name="index-fms_002dextensions-129"></a>Accept some non-standard constructs used in Microsoft header files.
|
||
|
|
|
||
|
|
<p>In C++ code, this allows member names in structures to be similar
|
||
|
|
to previous types declarations.
|
||
|
|
|
||
|
|
<pre class="smallexample"> typedef int UOW;
|
||
|
|
struct ABC {
|
||
|
|
UOW UOW;
|
||
|
|
};
|
||
|
|
</pre>
|
||
|
|
<p>Some cases of unnamed fields in structures and unions are only
|
||
|
|
accepted with this option. See <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed struct/union fields within structs/unions</a>, for details.
|
||
|
|
|
||
|
|
<p>Note that this option is off for all targets but x86
|
||
|
|
targets using ms-abi.
|
||
|
|
|
||
|
|
<br><dt><code>-fplan9-extensions</code><dd><a name="index-fplan9_002dextensions-130"></a>Accept some non-standard constructs used in Plan 9 code.
|
||
|
|
|
||
|
|
<p>This enables <samp><span class="option">-fms-extensions</span></samp>, permits passing pointers to
|
||
|
|
structures with anonymous fields to functions that expect pointers to
|
||
|
|
elements of the type of the field, and permits referring to anonymous
|
||
|
|
fields declared using a typedef. See <a href="Unnamed-Fields.html#Unnamed-Fields">Unnamed struct/union fields within structs/unions</a>, for details. This is only
|
||
|
|
supported for C, not C++.
|
||
|
|
|
||
|
|
<br><dt><code>-trigraphs</code><dd><a name="index-trigraphs-131"></a>Support ISO C trigraphs. The <samp><span class="option">-ansi</span></samp> option (and <samp><span class="option">-std</span></samp>
|
||
|
|
options for strict ISO C conformance) implies <samp><span class="option">-trigraphs</span></samp>.
|
||
|
|
|
||
|
|
<p><a name="index-traditional-C-language-132"></a><a name="index-C-language_002c-traditional-133"></a><br><dt><code>-traditional</code><dt><code>-traditional-cpp</code><dd><a name="index-traditional_002dcpp-134"></a><a name="index-traditional-135"></a>Formerly, these options caused GCC to attempt to emulate a pre-standard
|
||
|
|
C compiler. They are now only supported with the <samp><span class="option">-E</span></samp> switch.
|
||
|
|
The preprocessor continues to support a pre-standard mode. See the GNU
|
||
|
|
CPP manual for details.
|
||
|
|
|
||
|
|
<br><dt><code>-fcond-mismatch</code><dd><a name="index-fcond_002dmismatch-136"></a>Allow conditional expressions with mismatched types in the second and
|
||
|
|
third arguments. The value of such an expression is void. This option
|
||
|
|
is not supported for C++.
|
||
|
|
|
||
|
|
<br><dt><code>-flax-vector-conversions</code><dd><a name="index-flax_002dvector_002dconversions-137"></a>Allow implicit conversions between vectors with differing numbers of
|
||
|
|
elements and/or incompatible element types. This option should not be
|
||
|
|
used for new code.
|
||
|
|
|
||
|
|
<br><dt><code>-funsigned-char</code><dd><a name="index-funsigned_002dchar-138"></a>Let the type <code>char</code> be unsigned, like <code>unsigned char</code>.
|
||
|
|
|
||
|
|
<p>Each kind of machine has a default for what <code>char</code> should
|
||
|
|
be. It is either like <code>unsigned char</code> by default or like
|
||
|
|
<code>signed char</code> by default.
|
||
|
|
|
||
|
|
<p>Ideally, a portable program should always use <code>signed char</code> or
|
||
|
|
<code>unsigned char</code> when it depends on the signedness of an object.
|
||
|
|
But many programs have been written to use plain <code>char</code> and
|
||
|
|
expect it to be signed, or expect it to be unsigned, depending on the
|
||
|
|
machines they were written for. This option, and its inverse, let you
|
||
|
|
make such a program work with the opposite default.
|
||
|
|
|
||
|
|
<p>The type <code>char</code> is always a distinct type from each of
|
||
|
|
<code>signed char</code> or <code>unsigned char</code>, even though its behavior
|
||
|
|
is always just like one of those two.
|
||
|
|
|
||
|
|
<br><dt><code>-fsigned-char</code><dd><a name="index-fsigned_002dchar-139"></a>Let the type <code>char</code> be signed, like <code>signed char</code>.
|
||
|
|
|
||
|
|
<p>Note that this is equivalent to <samp><span class="option">-fno-unsigned-char</span></samp>, which is
|
||
|
|
the negative form of <samp><span class="option">-funsigned-char</span></samp>. Likewise, the option
|
||
|
|
<samp><span class="option">-fno-signed-char</span></samp> is equivalent to <samp><span class="option">-funsigned-char</span></samp>.
|
||
|
|
|
||
|
|
<br><dt><code>-fsigned-bitfields</code><dt><code>-funsigned-bitfields</code><dt><code>-fno-signed-bitfields</code><dt><code>-fno-unsigned-bitfields</code><dd><a name="index-fsigned_002dbitfields-140"></a><a name="index-funsigned_002dbitfields-141"></a><a name="index-fno_002dsigned_002dbitfields-142"></a><a name="index-fno_002dunsigned_002dbitfields-143"></a>These options control whether a bit-field is signed or unsigned, when the
|
||
|
|
declaration does not use either <code>signed</code> or <code>unsigned</code>. By
|
||
|
|
default, such a bit-field is signed, because this is consistent: the
|
||
|
|
basic integer types such as <code>int</code> are signed types.
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|